The Production of Electronic-Grade Silicon

Careful inspection of this unit, however, shows that NH3 is created from its constituent atoms, H and N, which enter the unit as H2 and N2. Matter is not created. Similar reasoning reveals that although nitrogen and hydrogen do not appear in the effluent, matter is not destroyed. There is chemical change in the reactor, but matter is neither created nor destroyed. 

Flowsheets are a graphical representation of a process. Creating a flowsheet is the first step toward producing architectural drawings for the process. As we shall learn, flowsheets are also used to analyze designs and compare competing designs. 

We will now create a flowsheet for a process to produce elemental silicon used in the manufacture of integrated circuits and other electronic devices. This seems easy - silicon is a nontoxic element, abundant in the Earth s crust, and it is stable at reasonable temperatures and pressures. However, the lack of a means to produce electronic-grade silicon once inhibited the mass production of solid-state electronic devices, such as transistors and diodes. The complication is the extraordinary purity needed. 

PROBLEM Produce elemental silicon with less than 1 part-per-billion impurities.  

To put this purity in perspective, consider that one part-per-billion (ppb) corresponds to one grain of salt in an entire railroad car of sugar. 

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Electronic Grade Silicon (EGS). As the first step in the production of electronic grade silicon (EGS), an impure grade of silicon is pulverized and reacted with anhydrous hydrochloric acid, to yield primarily tricholorosilane, HSiClg. This reaction is carried out in afluidizedbed at approximately 300°C in the presence of a catalyst. At the same time, the impurities in the starter impure silicon reactto form their respective chlorides. These chlorides are liquid at room temperature with the exception of vanadium dichloride and iron dichloride, which are soluble in HSiCl3 at the low concentration prevailing. Purification is accomplished by fractional distillation. 

Concerning eosts, silicon electronics is and will remain an expensive teehno-logy [5]. Production of electronic-grade silicon is an expensive process, as are the subsequent vacuum evaporation and lithography steps needed to make ehips out of the material. Due to the high costs, application of silicon in large-scale low-end eleetronics is not likely. 

Different techniques are currently used for the production of large scale electronic grade silicon. Metallurgical grade silicon powder at 300°C in the presence of a catalyst is reacted with anhydrous HC1 ... 

Whereas ca. 3000 t of ultrapure silicon ( electronic grade ) was produced in 1980 for the manufacture of electronic components markets, the booming electronic industry in the meantime has led to an explosive expansion in production capacity to ca. 20 10 t/a, of which 40% is in the USA, 30% is in Japan and ca. 30% is in Europe. Due to the strongly growing electronics market and the emerging photovoltaic market (solar cells on the basis of crystalline silicon), a strongly expanding demand for ultrapure silicon is expected in the future. 

The purity of silicon in this first step is only ca. 98%, and is referred to as metallurgical grade silicon (MG-Si). In order for the silicon to be used for electronics applications, additional steps are necessary to decrease the number of impurities. Reaction of MG-Si with hydrogen chloride gas at a moderate temperature converts the silicon to trichlorosilane gas (Eq. 3). When SiHCl3 is heated to a temperature of ca. 1,150°C, it decomposes into high-purity silicon and gaseous by-products (Eq.4). This reaction is typically performed in a bell-shaped Siemens-type reactor, where Si is deposited onto heated electrodes. 

The market for silicone elastomer products of this decade has expanded dramatically with increase of business machines and home electronics such as personal computers, copy machines, printers, TV sets, video cameras and so on. Various silicone products are used for electrical and electronic fields Based on the electrical conductivity, there are three grades of silicone rubbers. Insulating stocks are used for anode caps used for the cathode ray tube of TV sets, or plug boots and oil bleed connectors for automobiles, for instance. Electrically conductive silicones are used in products like rubber contact switches for TV remote controllers, handy phones, zebra connectors, EMI (electron magnetic interference) shield gasket and so on. And semiconductive silicone elastomers used for semiconductive rollers for plain paper copy machines and page printers, and antistatic materials. These relatively new materials have the volume resistivity of 10 -10 ohm-cm, and may be used in many other industrial fields. 

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